Printed circuit boards (PCBs) have long been formed as laminated structures upon which large numbers of devices such as integrated circuits are mounted or formed for use in a wide variety of electronic applications. Typically, these printed circuit boards have been formed with internal power and ground planes, or conductive sheets, the various devices including traces or electrical connections with both the power and ground planes for facilitating their operation.
Substantial effort has been expended in the design of such PCBs and the device arranged thereupon to compensate for voltage fluctuations arising between the power and ground planes in the PCBs, particularly for sensitive devices such as integrated circuits mounted or formed on the board surface and connected with both the power and ground planes for operation.
The voltage fluctuations referred to above are commonly caused by the integrated circuits switching on and off, the fluctuations resulting in "noise" which is undesirable and/or unacceptable in many applications.
A common solution to this problem in the past has been the provision of surface capacitors connected directly with the integrated circuits in some cases and connected with the power and ground planes in the vicinity of the selected integrated circuit in other cases. In any event, the surface capacitors were formed or mounted upon the surface of the PCB and connected with the respective devices or integrated circuits, etc. either by surface traces or by through-hole connections, for example.
Generally, surface capacitors have been found effective to reduce or, in other words, to smooth the undesirable voltage fluctuations referred to above. However, surface or bypass capacitors have not always been effective in all applications. For example, the capacitors themselves tend to affect "response" of the integrated circuits or other devices because they have not only a capacitive value but an inductive value as well. It is, of course, well known that inductance arises because of currents passing through conductors such as the traces or connectors coupling the capacitors with the devices or power and ground planes.
Furthermore, as was also noted above, the integrated circuits or other devices are a primary source of radiated energy creating noise from voltage fluctuations in the printed circuit boards. Different characteristics are commonly observed for such devices operating at different speeds or frequencies. Accordingly, the PCBs and device arrays as well as associated capacitors must commonly be designed to assure necessary noise suppression at both high and low speed operation.
In any event, the design of printed circuit boards and device arrays for overcoming these problems are well known to those skilled in the art of printed circuit board design. For purposes of the present invention, it is sufficient to realize that the use of surface mounted capacitors which are individually connected with the integrated circuits or devices substantially increase the complexity and cost of manufacture for the PCBs as well as undesirably affecting their reliability.
A novel approach to overcoming this problem and greatly simplifying the design and manufacture of printed circuit boards in order to provide capacitance for large numbers of integrated circuits or devices mounted thereupon was recently set forth in a copending U.S. patent application filed by John Sisler and assigned to Convergent Technologies, Inc. That copending application is incorporated herein by reference in order to assure a more complete understanding of the present invention and also to emphasize the importance of the Sisler concept within the present invention.
The Sisler concept overcame the problem of providing individual surface capacitors for large numbers of devices or integrated circuits on the PCB by making the PCB itself a capacitive element capable of providing a capacitive function for the various integrated circuits and/or devices. More specifically, the Sisler concept contemplated forming one or more capacitive layers internally within the PCB, the conductive sheets on the capacitive layers preferably forming the power and ground planes of the PCB.
In this manner, it became possible to commonly interconnect the individual devices and/or integrated circuits with both the power and ground planes as well as the internal capacitive element of the PCB by a single pair of traces or connectors.
Thus, the Sisler design provided a number of possible important advantages in PCB design. Initially, it entirely avoided the need for most if not all of the surface capacitors on the PCB. At the same time, with the capacitive layer also forming the power and ground planes for the PCB, the Sisler design reduced the number of electrical connectors associated with the integrated circuits and/or devices by approximately 50%.
The reduction or removal of these components from the PCB not only minimized manufacturing complexity and cost for the PCB but also greatly improved its reliability. Furthermore, removal of the surface capacitors from the PCB either made the device arrays upon the PCB less dense or permitted the addition of other surface devices or circuits.
In order to achieve all of the significant and desirable advantages referred to above, the Sisler design contemplated the need for assigning or allotting localized areas or portions of the internal capacitance layer or layers to each of the individual devices and/or integrated circuits. Thus, the Sisler design contemplated the need for capacitive layers with dielectric sheets and conductive sheets of greatly reduced thicknesses and/or very high dielectric constants generally beyond the capabilities of the existing state of the art.
The Sisler concept contemplated a number of approaches for achieving the necessary capacitive values because of the design parameters discussed above. For example, the Sisler concept required very thin dielectric sheets on the order of no more than about 0.5 mils with the dielectric material having a dielectric constant as high as 200.
Such characteristics were unavailable in the existing state of the art for dielectric materials. In addition, the contemplated need for ultra-thin capacitive layers also made the capacitive layers extremely fragile and difficult to work with, even in theory.
Other techniques were also considered for overcoming these design problems. For example, consideration was given to etching of a relatively thin layer of dielectric material in order to substantially increase its surface area followed by sputtering and/or electrodeposition of conductive metal onto the dielectric material in order to achieve capacitor characteristics as needed. However, none of the approaches discussed above was found to be capable of providing a satisfactory solution to design of the internal capacitive layer for such a capacitive printed circuit board.
Accordingly, although the Sisler concept offered numerous substantial advantages in manufacturing ease and reduced cost as well as increased reliability, at least in theory, there remained a need for an effective capacitive layer to produce a working embodiment of the capacitive PCB of the Sisler concept.